Interpretation of Laboratory Results and Management
Primary Diagnosis: Overt Hypothyroidism with Iron Deficiency Anemia
This patient has overt hypothyroidism requiring immediate levothyroxine therapy. The TSH of 5.77 mIU/L combined with low free T4 (0.73 ng/dL, assuming normal range ~0.8-1.8) definitively indicates primary hypothyroidism 1. The constellation of symptoms—fatigue, cold intolerance, and hair loss—are classic manifestations of hypothyroidism 2, 3.
Laboratory Interpretation
Thyroid Function:
- TSH 5.77 mIU/L: Elevated above the normal reference range (0.45-4.5 mIU/L), indicating inadequate thyroid hormone production 1
- Free T4 0.73 ng/dL: Below normal range, confirming overt hypothyroidism rather than subclinical disease 1
- Total T4 5.7 mcg/dL and T3 uptake 24%: These values are consistent with hypothyroidism 4
Hematologic Findings:
- Hemoglobin 11.0 g/dL, MCV 76.5 fL, MCH 23.8 pg, MCHC 31.1%: These values indicate microcytic hypochromic anemia, most consistent with iron deficiency 1
- RDW 16.7%: Elevated, supporting iron deficiency as the cause of anemia 1
- The anemia may be partially related to hypothyroidism itself, which can impair erythropoiesis, but the microcytic pattern strongly suggests concurrent iron deficiency 3
Immediate Treatment Recommendations
Levothyroxine Initiation
Start levothyroxine at 1.6 mcg/kg/day for patients under 60 years without cardiac disease, or 25-50 mcg/day for patients over 60 or with cardiac risk factors 1. The full replacement dose rapidly normalizes thyroid function and prevents cardiovascular dysfunction, adverse lipid profiles, and quality of life deterioration 1.
Critical safety consideration: Before initiating levothyroxine, rule out concurrent adrenal insufficiency, as starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis 2, 1. However, the elevated TSH with low free T4 pattern indicates primary (not central) hypothyroidism, making adrenal insufficiency less likely 1.
Monitoring Protocol
Recheck TSH and free T4 in 6-8 weeks after initiating therapy 1. This interval is critical because it represents the time needed to reach steady state 1. Target TSH should be within the reference range of 0.5-4.5 mIU/L with normal free T4 levels 1.
Once adequately treated, repeat testing every 6-12 months or if symptoms change 1. Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, and cardiac complications 1.
Essential Follow-Up Laboratory Testing
Confirm Diagnosis and Assess Etiology
Measure anti-thyroid peroxidase (TPO) antibodies 2, 1. Positive TPO antibodies confirm autoimmune thyroiditis (Hashimoto's disease) as the etiology and predict higher risk of progression, with 4.3% annual progression to more severe hypothyroidism versus 2.6% in antibody-negative individuals 1, 5.
Evaluate Iron Deficiency
Order serum ferritin, iron, total iron-binding capacity (TIBC), and transferrin saturation 1. The microcytic anemia pattern strongly suggests iron deficiency, which requires separate treatment. Iron supplementation should be taken at least 4 hours apart from levothyroxine, as iron binds to levothyroxine and prevents absorption 6.
Screen for Associated Conditions
Check vitamin B12 levels 1. Patients with autoimmune thyroid disease should be screened periodically for vitamin B12 deficiency, as autoimmune conditions often cluster 1.
Consider screening for celiac disease if iron deficiency is confirmed 1. Celiac disease can cause both iron malabsorption and is associated with autoimmune thyroiditis 1.
Common Pitfalls to Avoid
Do not delay treatment waiting for additional testing 1. With overt hypothyroidism (elevated TSH and low free T4), levothyroxine should be initiated immediately 1.
Never start iron supplementation at the same time as levothyroxine without proper spacing 6. Phosphate binders including ferrous sulfate bind to levothyroxine and must be administered at least 4 hours apart 6.
Avoid attributing all symptoms solely to hypothyroidism 7. The iron deficiency anemia is a separate condition requiring concurrent treatment and may contribute significantly to the fatigue 1.
Do not over-treat 1, 8. Even slight levothyroxine overdose carries significant risk of atrial fibrillation and osteoporotic fractures, especially in elderly patients 1. Target TSH should remain within the normal reference range of 0.5-4.5 mIU/L 1.
Expected Outcomes
Hair loss should improve within 3-4 months of adequate levothyroxine replacement 1. Levothyroxine should stabilize weight and improve energy within 6-8 weeks 1. However, if symptoms persist despite normalized TSH, reassess for other causes including inadequately treated iron deficiency 3.